Transcript Thermal_Vs_Non-thermal

Food Microbiology 1
Unit 5
Thermal and Non-Thermal
Preservation
Thermal
• Pasteurization
• Commercial Sterilization
Non-thermal
• Low Temperature
• Irradiation
• Chemical
• Micro filtration
• High Pressure
• Pulsed electric field
Thermal (High Temperature) Processing
• Logarithmic Death: Microbial destruction by
heat occurs in a logarithmic fashion allowing
us to predict the death of a population of
organisms.
• The theory of logarithmic death is based on a
single hit or one event equals death
Pasteurization
• Derives its name from the mild heat
treatments developed by Louis Pasteur to
prevent or delay spoilage of wine and beer
• Today it refers to a heat process that results
in destruction of all vegetative cells (nonspore formers) of pathogens expected in that
food
Pasteurization
• The process of pasteurization is based on
food safety and not on food preservation
alone
 It kills target pathogens
 Extends shelf life ( shelf-life refers to the
amount of time from packaging of the food
product to the time of spoilage under appropriate
storage conditions).
 Does not inactivate all microbes present
 Pasteurized food usually requires additional
control measures (such as refrigeration, low aw,
low pH) to prevent rapid spoilage
Pasteurized Foods
• The most common pasteurized food is milk
• Originally designed to eliminate
Mycobacterium tuberculosis and Coxiella
burnetti
• Fruit juice
• Spoilage yeast and bacteria, E. coli
O157:H7
• Beer
• Spoilage bacteria and yeast
Pasteurized Foods
• Liquid egg
• Salmonella and spoilage bacteria
• Honey
• Spoilage yeast
• Meat surfaces (steam, hot water)
• E. coli O157: H7, Salmonella,
Campylobacter
Milk Pasteurization
Time/Temperature Combinations
• High Temperature Short Time (HTST) 15 sec @
72oC
• Low Temperature Long Time (LTLT) 30 min at
63oC
• Heat treatments are established on the
basis of safety first (elimination of
pathogens) and spoilage (extension of shelf
life) second.
• Applying high temperatures over a short
time preserves the sensory and nutritional
quality of milk
• Other combinations may result in a sensory
quality not accepted by consumers
• Can effect the quality of products derived
from treated milk (e.g. cheese)
Commercial Sterilization
• Some milk is sold in cans (evaporated or
sweetened condensed milk) or in boxes that
remain at room temperature
• The boxed milk is known as Ultra High
Temperature milk (UHT) milk
• UHT milk has undergone commercial
sterilization and so can be stored at room
temperature
• UHT treatment is 2 sec @ 140-150oC
• Sterilization: Inactivation of all
microorganisms
Essential in clinical settings (surgical
instruments)
• Commercial Sterilization: “ A product is not
necessarily free of all microorganisms, but
those that survive the sterilization process
are unlikely to grow during storage and
cause spoilage”
Commercial Sterilization
• A product that has undergone commercial
sterilization is free of vegetative and sporeforming pathogens and spoilage
microorganisms that are capable of growing
in that food under typical non-refrigerated
storage conditions
• Most common commercially sterilized foods
are canned products
Commercial Sterilization
• Primary Objective:
Destroy the most heat resistance
pathogenic spore-forming organismsClostridium botulinum
• Secondary Objective:
Destroy vegetative and spore-forming
microorganisms that cause spoilage.
Spoilage spore-formers are usually more
heat resistant than pathogenic spore formers
Thermal Destruction Curves
• Thermal destruction curves provide an
empirical model to calculate time/temperature
relationships used in processing
• D value
• Z value
• F value
D -value
D-value- Decimal Reduction Time: Is the time
needed to reduce a population of microorganisms
by 90% (1 log cycle) at a specified temperature
and in a specified medium
• If the initial population was 100 CFU/ml
10 CFU/ml would remain after a 1 log cycle
reduction
D -value
105
D-value
Time (s) @ 121oC
104
D –value Formula
DT Value = t2-t1/ (log N0-log N1)
T= temperature
t1= initial time
t2= final time
N0= initial population
N1= final population
From previous example:
D121= 45-30/5-4
= 15/1= 15 sec
Z- Value
Z-value: is the change in temperature required
to produce a 10-fold change (1 log) in D-value.
Z-values are calculated from the slope of the
curve of D-value vs temperature
Z- value is the measurement of the sensitivity
of an organism to changes in temperature
Z- Value
Z
D-value
Z- Value Formula
Z = T2 – T1/ log a- log b
T2= Final temperature
T1= Initial temperature
a = upper D-value
b = lower D-value
From previous figure:
Z= 240-220/log 100- log 10
Z= 20/2-1
Z= 20oF
Exercise
D value determination for E. coli O157:H7 in beef at
60oC: Calculate the D value of the organism under these
conditions
Time (min)
Log10viable count
(cfu/g)
0.2
7.1
0.5
6.5
1.0
6
Calculate the Z value of the organism
Temperature (oC)
Log10 D value (min)
55
60
0.75
-0.7